1. Field of the Invention:
The present invention relates to an austenitic chromium-manganese-nickel-nitrogen stainless steel having a relatively low nickel content, which exhibits a unique combination of properties enabling its use in cryogenic applications requiring low temperature toughness and good resistance to cracking in chloride corrosive media where resistance to stress corrosion is needed, and in the production of cold reduced articles which have great tensile strength and low magnetic permeability. While not so limited, the alloys of the invention have particular utility in the fabrication of welded pressure vessels for cryogenic service requiring high strength at room temperature together with good cryogenic toughness and stability.
2. Description of the Prior Art:
Periodic scarcities of nickel, and its relatively high cost, have stimulated workers in the art to develop nickel-free or low nickel austenitic alloys. Over a period of years many such alloys have been investigated.
An article by R. Franks, W. Binder and J. Thompson (A.S.M. Transactions, Vol. 47, pp. 231-266, 1955) discloses the structural constitution of steels containing about 0.1% carbon and about 0.15% nitrogen at chromium levels of 12 to 18%, 0 to 22% manganese and 0 to 14% nickel. It was there concluded that a fully austenitic structure cannot be produced with manganese alone at chromium levels above about 15%, despite the fact that carbon and nitrogen expand the austenitic region in steels of this composition range and are more potent in this respect than nickel.
U.S. Pat. No. 2,778,731, issued Jan. 22, 1957 to D. J. Carney discloses an austenitic low nickel alloy steel consisting of from 17% to 18.5% chromium, 14% to 20% manganese, 0.05% to 1.00% nickel, 0.06% to 0.15% carbon, 0.25% to 1.0% nitrogen, 0.25% to 1.0% silicon, and balance iron. The steel of that patent was alleged to have high strength and a high work-hardening rate and to be comparable in mechanical properties to AISI Types 301 and 302. The steel is currently sold by U.S. Steel Corporation under the registered trademark TENELON, having the following composition range: chromium 17.0-19.0%, manganese 14.5-16%, nickel 0.75% maximum, carbon 0.08-0.12%, nitrogen 0.35% minimum, silicon 0.75% maximum and remainder iron.
Reference may also be made to U.S. Pat. No. 2,820,725 issued Jan. 21, 1958, to R. D. Wasserman et al.; U.S. Pat. No. 3,151,979 issued Oct. 6, 1964 to D. J. Carney et al; and U.S. Pat. No. 3,192,041 issued June 29, 1965, to J. J. Kanter et al.
Among other high-strength low nickel austenitic alloys which have been developed are a Type 16-16-1 and Allegheny-Ludlum Type 205. Type 16-16-1 contains from 15% to 16% chromium, from 16% to 17.5% manganese, less than 1% nickel, 0.10% carbon, 0.20% maximum nitrogen, 0.20% to 0.70% silicon, and remainder substantially iron. Allegheny Type 205 contains from 16% to 18% chromium, 14.0% to 16.0% manganese, 1.1% to 2.0% nickel, 0.12% to 0.25% carbon, 0.32% to 0.40% nitrogen, 0.2% to 0.7% silicon, and remainder substantially iron.
It has been previously believed that if manganese is substituted for nickel as an austenite former, the substitution should be made on a 2:1 basis. An article in Metal Progress Data sheet, February 1960, page 100-B, by A. Schaeffler, contains a constitution diagram in which manganese is considered to be one-half as potent as nickel as an austenite former in high-chromium alloys containing about 0.10% nitrogen.
Present standard austenitic stainless steels used for cryogenic service include AISI Types 304, 304-N, 316, Armco 2-6-9, and most recently U.S. Steel Crogenic TENELON.
Although Type 304 has good toughness at cryogenic temperatures, it transforms to untempered martensite when deformed and hence has poor fatigue life. Its melting specifications call for from 8.00 to 10.50% nickel, and hence it is relatively expensive. Moreover, it has relatively low room temperature tensile and yield strengths (about 85 and 35 ksi, respectively, in the annealed condition), and the design stresses must thus be kept at low values.
Type 304-N is somewhat stronger than Type 304 at room temperature but is still subject to transformation to martensite at cryogenic temperatures, which tends to a lower fatigue life.
Type 316, because of its high nickel content (from 10% to 14%), is more stable against transformation to martensite at cryogenic temperatures, which is conducive to good fatigue life, but use of the alloy severely limits design stresses because of its low strength at room temperature.
Armco Type 2-6-9 is stable against transformation to martensite at cyrogenic temperatures, but also has the disadvantage of being expensive because of a relatively high nickel content.
Cryogenic TENELON (which has a nominal composition of 0.08% carbon, 16% manganese, 18% chromium, 5.5% nickel, 0.38% nitrogen and remainder substantially iron) achieves good room temperature strength levels and stability against martensite transformation at low temperatures, but has the disadvantages of a relatively high nickel content, poor weldability, and poor stress corrosion resistance. It also has substantially lower impact values at cryogenic temperatures than Type 304. Data on cryogenic TENELON are given in a reprint of a paper by C. E. Spaeder, Jr., et al., in Metals Engineering Quarterly, ASM, August 1969, pages 1-15.
Stress corrosion resistance is another important property of stainless steels in many applications, such as formed and/or welded vessels used in chemical processing. The residual stresses in such vessels or other products cannot be relieved by annealing because of their size, and the stresses are often high enough to cause cracking in certain environments. All the previously mentioned prior art alloys having nickel contents of about 8% suffer from stress corrosion cracking when exposed to hot-chloride-containing media while under stress.